a) Field of the Invention
The invention is directed to a method for generating a predetermined break line in a multilayer airbag covering in which the material of a carrier layer has a greater density than the material of an adjoining supporting layer adjoined by a decorative layer. A generic method of this kind is known from DE 102 27 118 A1.
b) Description of the Related Art
Many methods are known for introducing a predetermined break line in a multilayer airbag covering. Initially, only the dashboard or steering wheel hub were used as airbag coverings for covering a front airbag. In the meantime, it has become standard also for door panels and seat upholstery to cover a side airbag, for the inside roof lining to cover a head airbag, or even for the safety belt to cover a front airbag, e.g., for the rear passengers.
This has increased not only the variety of airbag covering constructions but also the variety of materials that are used to produce multilayer airbag coverings for this purpose. Currently, the most common layer construction for an airbag covering comprises a rigid carrier layer, e.g., of plastic or natural bonded fiber, a soft supporting layer, e.g., of foamed material or a spacer fabric, and a decorative layer, e.g., of plastic, woven textile or leather. In a layer construction of this kind, the material density of the carrier layer is appreciably greater than that of the supporting layer.
Although it is not expressly mentioned in all of the relevant publications, a predetermined break line with a defined tear resistance must be produced in principle and should be invisible from the passenger compartment (decorative side of the airbag covering) for aesthetic reasons.
The relevant prior-art methods for producing a predetermined break line of the type mentioned above differ substantially with respect to the sequence of individual method steps on one hand and the application of different cutting techniques on the other hand.
With regard to the technical sequence, the methods can be grouped according to whether the layer construction of the airbag covering is produced first and then a predetermined break line is introduced in the prefabricated airbag covering, or whether a predetermined break line is introduced in individual layers before these layers are assembled.
The different cutting techniques are essentially defined by the application of different tools. Mechanical cutting tools or chip-removing tools, heat knives, ultrasonic knives, and lasers are used for this purpose.
In recent years, laser methods in particular have progressed and expanded in application. For a layer construction of the type described above, lasers are especially advantageous in that no mechanical pressure is exerted on the workpiece (in this case, the airbag covering). Further, the tool is not subject to wear, which is particularly beneficial for large-scale production as in the automobile supplier industry. Further, it is advantageous that different ablation regimes which may be advantageous for different material compositions can be realized in a simple manner by selecting suitable laser parameters such as laser output and pulse frequency. Further, ablation can be regulated by detecting the working beam transmitted to the ablation site through the residual material or when there is an opening in the material.
In all of the known prior-art laser methods in which a predetermined break line is introduced in a prefabricated airbag covering having a layer construction of the type described above, a laser beam is directed to the airbag covering on the carrier layer side and is moved long the desired predetermined break line relative to the airbag covering. It is known to select the type of laser beam and its wavelength, the laser output, the relative speed, pulse duration, and pulse frequency depending on the layer construction and to regulate the variable laser parameters depending on the ablation depth or residual wall thickness.
As was already mentioned, the predetermined break line must have a reproducible tear resistance defined along its length. The tear resistance should be low enough so that, on the one hand, the predetermined break line can be destroyed by only a slight tearing force in case the airbag is activated and, on the other hand, so that the predetermined break line does not break already due to an uncontrolled random force acting on the passenger compartment side. An ablation regime is selected depending on a correspondingly suitable tear resistance and the material characteristics and material thickness of the individual layers. The remaining webs of material in the different layers, their widths and spacing, and the ablating depth determine the tear resistance along the predetermined break line.
Aside from a suitable reproducible tear resistance, it must also be ensured that the predetermined break line remains invisible over the long term. On the one hand, this means that the decorative layer may not be overly weakened by too great an ablation depth, and on the other hand the supporting layer must be retained as far as possible.
To solve this problem, it is known, for example, from DE 196 36 429 C1, to generate the weakened line by means of a series of blind holes. The blind holes extend completely through the carrier layer and the supporting layer into the decorative layer leaving a remaining residual wall thickness. Instead of blind holes, a satisfactory predetermined break line can also be generated by means of microperforations which are not perceptible to the naked eye.
However, practical experience has shown that these blind holes or microperforations have a nearly constant diameter only in the region of the carrier layer. In the region of the supporting layer, the blind holes undergo a distinct bubble-like expansion. The increased ablating volume in the supporting layer can be explained particularly in that the material density is substantially lower than in the carrier layer. In addition to the evaporation caused by the laser, the hot combustion gases also promote evaporation of the material. The combustion gases which can only escape in limited quantity via the opening of the respective blind hole in the carrier layer cause extensive displacement of the supporting layer due to their pressure combined with their temperature which accelerates the softening of the supporting layer.
Accordingly, in order to obtain webs in the supporting layer with an effective minimum width between the individual blind holes, there must be a defined minimum distance between the centers of the holes that is greater than the maximum diameter of the blind holes in the region of the supporting layer. For decorative layers with a high tear resistance, this distance may be too great for generating a weakened line with the desired tear resistance.
As a solution to this problem, the Applicant describes in Patent Application DE 102 27 118 A1 how groups of blind holes of different depth are deliberately generated. A first group extends only in the carrier layer so that the supporting layer lying above the latter is retained and a wide web is formed as a support for the decorative layer. A second group penetrates the supporting layer into the decorative layer. The distances between the hole centers can be selected so as to be small enough that webs are only retained in the decorative layer. Regardless of the distance, the supporting layer is destroyed in this ablation regime. This means that a smaller spacing causes a greater weakening of the decorative layer without affecting the supporting action of the supporting layer.
The weakening may also be unsatisfactory in this method when the tear resistance of the decorative layer is very high.